If the Morganza Floodway is not opened to funnel 300,000 cubic feet per second of water from the Mississippi River into the Atchafalaya River basin, the additional water could cause levees to fail along the river from Morganza to Plaquemines Parish, including all of the New Orleans area, resulting in as much as 25 feet of floodwater, according to a map provided to state officials by the Army Corps of Engineers on Tuesday.

The worst-case scenario obtained by The Times-Picayune is part of the risk assessment conducted this week by corps officials in support of opening the floodway. The combination of water from the floodway and from the Old River Control Structure just upriver of Morganza pouring into the Atchafalaya basin will flood a large swath of mostly unpopulated land.

It will also threaten Morgan City, Houma and several smaller communities.

Ironically, much of the Atchafalaya Basin would still flood if the spillway is not opened, according to the new map, because the Old River Control Structure will be sending twice as much water from the Mississippi into the basin as it normally does. Meanwhile, the river will still overtop the relatively low Morganza floodway structure even if it's not opened, and several other levee failures could occur between Morganza and Baton Rouge.

Corps Maj. Gen. Michael Walsh, president of the Mississippi River Commission and commander of all corps districts along the river, has strongly hinted that he will approve opening the Morganza Floodway sometime between Friday and Tuesday. That's when the rate of water moving past Red River Landing, across from the Louisiana State Prison at Angola, will reach 1.5 million cubic feet per second, which is the official trigger for opening the spillway.

During a news conference in Baton Rouge, Gov. Bobby Jindal said he expects Walsh to order the opening of the spillway. He urged residents in affected areas to prepare for evacuations if called for by their local elected leaders.

"Based on various inundation maps, you're looking at roughly 3 million acres that will be impacted, be underwater," when the floodway opens, Jindal said. "That includes about 18,000 acres of cropland just within the Atchafalaya basin."

According to census data, about 2,500 people inside the floodway, including residents of Melville, Butte Larose and Krotz Springs, could be surrounded by water, and another 22,500 people and 11,000 structures will be affected by some flooding, Jindal said.

Jindal said it will take about three days from the opening of the floodway until the first water from Morganza reaches Morgan City, which already is experiencing high water from the Old River Control Structure.

Serious flooding also is expected in parts of northern Louisiana as rivers and streams that normally flow unimpeded into the Mississippi have begun to back up. No rivers or streams enter the Mississippi in south Louisiana.

Too close for comfort

The National Weather Service has predicted that at its crest, 1.9 million cubic feet per second of water will flow past the Red River Landing if the floodway is not opened.

That would result in a crest of 19.5 feet at the Carrollton Gauge in New Orleans, which is 2.5 feet above official flood stage and just 6 inches below the top of floodwalls.

The new corps map assumes that such a high water level could result in multiple failures of earthen levees, floodwalls or other structures along the river, said Walter Baumy, chief of engineering for the corps' New Orleans office.

The record high water levels also would cause a dramatic disruption to business in the Port of New Orleans and elsewhere along the river in the New Orleans area, said Bob Turner, executive director of the Southeast Louisiana Flood Protection Authority-East.

"With a stage at Carrollton of 19.5 feet, we would have to close most of the floodgates along the Mississippi River levee within our jurisdiction," he said. "The port, Public Belt Railroad, recreational areas like the Audubon Aquarium and the Butterfly Park would have to close."

More significant, though, would be the unknown effects on the levees, Turner said.

"We have a good bit of experience dealing with seepage and sand boils, saturated soil conditions when the river stages are around 17 feet," he said. "Once the river gets over 17.5 feet, I don't know that we've got a lot of experience dealing with those same issues. Seepage and sand boils, that uncertainty gives us some concern. My gut is if it gets higher, it's going to be more difficult to keep those things under control."

Even if the river doesn't go much over 17 feet at New Orleans, a level reached at noon Tuesday, levee officials will be keeping careful watch when the river begins to drop, now not expected until mid-June, Turner said.

"If the river begins to fall rapidly, we could have incidents where saturated levees slide into the river," he said.

Laying bare the risks

Baumy emphasized that the no-Morganza map was not designed with the public in mind, but rather to help corps engineers plan better.

"The maps were done in a day or two, as we were trying to assess risk to show, here's the options on the table and here's some things to think about," he said.

For instance, while the color code on the map shows the possibility of 20 to 25 feet of water occurring from Kenner through New Orleans if levees were to fail, elevations vary dramatically within that area.

And while the map indicates flooding all along the east side of the river from Simmesport to New Orleans, it is not meant to indicate all of that area would be flooded, Baumy said. Rather, the colors indicate the highest water level possible if the levee were overtopped or breached in those areas. The corps would not expect all segments of the levee system to fail, he said.

State agencies already are scrambling to prepare for flooding in the Atchafalaya basin and in northern flood areas, Jindal said, saying 500 additional National Guard troops have been deployed.

The National Guard has set up liaison teams in 19 parishes and assigned troops to work with corps teams and the Coast Guard.

The state also is rounding up thousands of feet of Hesco basket, fiber and metal baskets that are filled with rock and dirt to create temporary levees.

Evacuation guidelines

Spillway fly over Tuesday, May 10, 2011The Army Corps of Engineers opened an additional 44 bays of the Bonnet Spillway structure Tuesday, May 10, 2011 to divert water from a rapidly rising Mississippi River. The spillway was last opened in 2008 for 28 days.

Catahoula, Point Coupee and Iberia parishes have begun issuing evacuation orders from some localities, while others, including St. Landry, St. Mary, St. Martin, West Baton Rouge, Assumption and Ascension, have issued voluntary evacuation recommendations to residents and businesses in expected flood areas.

Jindal also is lobbying federal officials to upgrade their treatment of the potential flooding as a national disaster. FEMA has agreed to provide direct federal assistance to 22 parishes, but the disaster agency has not agreed to let the state bill the federal government for its costs in flood fighting.

In a Monday letter to President Barack Obama, Jindal said the state has estimated its costs for the first 30 days to be at least $80 million, including $20 million the Department of Child and Family Services will need for shelters.

Jindal also has asked the Defense Department to reimburse the state for National Guard mobilization because the Morganza Floodway is part of the national floodway system.

Agriculture & Forestry Commissioner Mike Strain also asked the U.S. Department of Agriculture on Tuesday to consider the floodway opening to be a natural disaster, which would make farmers eligible for federal assistance.

Mississippi River flooding update with Mark SchleifsteinTimes-Picayune environmental reporter Mark Schleifstein gives the latest update on the status of the Mississippi River and explains when and why the Morganza floodway will be opened within the next several days in this video.

Meanwhile, at Montz, an additional 44 bays of the Bonnet Carre Spillway were opened Tuesday, increasing the number of bays opened to 72. The spillway has 350 bays.

The action will increase the flow of water to about 70,000 cubic feet per second, according to spillway manager Chris Brantley. Another 38 bays could be opened today.

When fully open, the spillway diverts 250,000 cubic feet of water per second into Lake Pontchartrain.